1. Field of the Invention
Implementations of various technologies described herein generally relate to geophysical data processing.
2. Discussion of the Related Art
This section is intended to provide background information to facilitate a better understanding of various technologies described herein. As the section's title implies, this is a discussion of related art. That such art is related in no way implies that it is prior art. The related art may or may not be prior art. It should therefore be understood that the statements in this section are to be read in this light, and not as admissions of prior art.
Seismic exploration is conducted on both land and in water. In both environments, exploration involves surveying subterranean geological formations for hydrocarbon deposits. A survey typically involves deploying acoustic source(s) and acoustic sensors/receivers at predetermined locations. The sources impart acoustic waves into the geological formations. Features of the geological formation reflect the acoustic waves to the sensors. The sensors receive the reflected waves, which are detected, conditioned, and processed to generate seismic data. Analysis of the seismic data can then indicate probable locations of the hydrocarbon deposits.
However, not all of the acoustic waves propagate downward into the geological formation. Some of the acoustic waves are “interface waves” that propagate along an interface between two media instead of through a medium. An interface wave can travel at the interface between the Earth and air—e.g., surface waves—, between the Earth and a body of water—e.g., Scholte waves—, or between a shallow interface within the near-surface of the Earth—e.g., refracted shear waves. Surface waves or seismic waves that propagate laterally through the near surface of the Earth often create a “ground roll” in acquired seismic data. Ground roll is a type of coherent noise generated by a surface wave that can obscure signals reflected from the geological formation and degrade overall quality of the seismic data resulting from the survey. Consequently, most surveys attempt to eliminate, or at least reduce, ground roll. In the following, the term, ground roll, will be used to refer to surface waves, as is common in the exploration industry.
Techniques for mitigating ground roll include careful selection of source and geophone arrays during the survey and filters and stacking parameters during processing. However, because the ground roll can be heavily (back)scattered by near-surface heterogeneities, conventional frequency and wave number (“FK”)-filtering techniques are often unsuccessful: the noise is distributed over a large range of (out-of-plane) wave numbers outside the expected FK-slice in a manner that is difficult to predict without highly detailed knowledge of the near-surface scatterers.
The phenomenon of interface waves is described above in the context of seismic surveying. However, their existence is not limited to that technology. The phenomenon may also be encountered in electromagnetic surveying or non-destructive testing (i.e., geophysical data), for instance. Interface waves raise similar concerns and have similar effects on the efficacy of these technologies as well.